Ultrafast superconducting digital circuits are based on Josephson junctions integrated together according to RSFQ Logic (Rapid-single-flux-quantum), as originally developed and described by K. K. Likharev and V. K. Semenov (1991). Fast memory circuits in the same technology are also required for most non-trivial digital applications. One class of memory arrays are random-access memories, or RAM, which are particularly important for digital computing applications. Such applications require equally fast data writing and data retrieval. This is in contrast to many digital signal processing applications, in which the memory contents need to be read out quickly, but updated only rarely, requiring a programmable read-only memory (PROM) instead of a RAM. A particular application of interest is a circuit for real-time digital predistortion of radio-frequency (RF) signals, where the predistortion parameters would be maintained in a digital lookup table.
There have been several circuits proposed for superconductor RAM, such as the Ballistic RAM circuit invented by Herr (U.S. Pat. No. 6,836,141). However, such a circuit does not help one design a fast PROM, which has an architecture that is completely different. There have been no prior publications or patents describing a PROM-type RSFQ memory array or lookup table.
The article by Bunyk et al., entitled RFSQ Microprocessor: New Design Approaches in IEEE Transactions on Applied Superconductivity, Vol. 7, No. 2, June 1997, pp 2697-2704 utilizes an RFSQ Data Processing pipeline architecture, similar but distinct from that used in the code matching network of the present invention.